The grade II listed site, which features 200-year-old Himalayan rhododendron trees, is registered as a site of special scientific interest. But violent storms in February felled scores of centuries-old trees, crushing many rare and unusual plants that sheltered beneath them.
A garden restoration and replanting plan was put in place by garden consultant Tony Russell.
Plas Tan y Bwlch head gardener Chris Marshall said: "It was like a scene out of a disaster movie. As a result, because of safety issues, we had to stop the public from accessing the gardens."
The venue's head of business Andrew Oughton added: "Seeing the culmination of all the replanting work will be a very interesting experience."
The Victorian estate was the home of the Oakeley family, who owned the slate quarry at Blaenau Ffestiniog. The mansion is set in parkland and woodland.